The present invention relates to a system and a method for determining the process viscosity (xcexc) of a fluid in a film metering device. The process viscosity (xcexc) is calculated using operating parameters and measurements made on the film metering device while it applies a coating layer on a substrate, such as a paper web or any other suitable flexible material. This invention allows the process viscosity (xcexc) of the coating fluid to be continuously monitored.
The paper coating industry is one area where film metering devices are used. A common film metering device in that industry is the metering-size press, which has been found to be flexible in dealing with a wide range of coating fluids of various specific weights and kinds of paper webs. FIG. 1 shows a typical example of a metering-size press (10) as found in the prior art. It comprises a transfer roll (12) and a proximate backing roll (14), both having the same tangential speed but rotating in opposite directions. The tangential speed (Vt) of these rolls (12,14) is typically from 50 to 3000 m/min. The transfer roll (12) and the backing roll (14) are pushed one against the other with a preset load. The coating fluid (22) is carried between the rolls (12,14) and picked up by a paper web (20) wound around the backing roll (14). The paper web (20) picks up most of the coating fluid (22) adhering on the transfer roll (12) and forming a coating film (24).
The fluid (22) is supplied to the transfer roll (12) through a feeding chamber or box (26) having a transversal length (W) that substantially corresponds to that of the film (24). The thickness of the film (24) is controlled by means of a rigid metering rod (30), usually chrome-plated, rotating in the same direction than that of the transfer roll (12) but at a much lower tangential speed (Vm), typically 60 m/min or less. Its diameter is small compared to that of the transfer roll (12). The excess of coating fluid (22) is scraped off from both the metering rod (30) by a blade (not shown).
The metering roll (30) is pushed against the transfer roll (12) with a preset load. The presence of the coating fluid (22) creates a thin space between the transfer roll (12) and the metering rod (30), which space is referred to as the metering nip (32). The minimum gap in the metering nip (32) is in fact extremely small, of the order of 10 to 100 xcexcm. Such small gap is difficult to obtain with a pair of hard rolls, especially on a full-size metering-size press (10) which can be up to 8 meters wide. In practice, the transfer roll (12) is usually covered with a soft elastomeric outer layer that reduces the risks of clashing with the metering rod (30) and allows more tolerance on the runout. The minimum gap results from an equilibrium between the deformation of the fluid and the elastic forces due to the deformation of the elastomeric outer layer. Nevertheless, the transfer roll (12) can also be provided with a rigid outer surface, such as a chrome-plated surface. The flow pattern in a metering nip is characterized by a deformation of the fluid, resulting from a combination of shear and extension. It is also characterized by a deformation rate that reaches very high values. For example, the deformation rate can be as high as 106 sxe2x88x921 in a metering nip whose minimum gap is 20 xcexcm and with a transfer roll having a tangential speed (Vt) of 2000 m/min.
The viscosity is a material function of a fluid. It is measured in well defined standard flow conditions. Two viscosities can be defined, according to the type of deformation imparted to the fluid, namely the shear viscosity and the elongational or extensional viscosity. The shear viscosity of a given fluid can be obtained using commercial rheometers, such as rotational rheometers with cone and plate geometries, rheometers with coaxial cylinders, or capillary rheometers. Measurements are done in well established permanent flow conditions or oscillatory flow conditions. Usually, corrections must be done to compensate end effects, wall effects or temperature effects. Unlikely, the extensional viscosity is usually difficult to measure since purely extensional flows cannot be easily generated. It is known from the theory that Newtonian fluids have an extensional viscosity which is three times the shear viscosity.
Real flows are generally quite different from standard flow conditions. In a process, a fluid is submitted to a combination of shear and extension that may vary in time. As a result, the viscosity of the fluid changes as well. The fluid behavior is more accurately characterized in terms of a process viscosity, which refers to the viscosity of a fluid in a given location of the process and under given operating conditions. The problem is that the only accurate way to measure the process viscosity is at the given location and under the real operating conditions. However, this has proved hitherto to be very difficult to achieve.
Although in-situ measurements of the behavior of the fluid were not possible, some techniques were devised to provide some ways of controlling the quality of the fluid to be used in a process. For instance, the Brookfield viscosity is measured with a spindle rotating at low speed in a small container filled with the fluid under investigation. There is also the Hercules viscosity, where the fluid is set between two concentric cylinders having non-identical rotation speeds. Measurements are made while the rotation speeds of the cylinder are continuously accelerating and decelerating. These two measurements may be used as reference values.
A concentrated suspension can be defined as liquids having a solid percentage of 30% or more by weight. The internal structure of concentrated suspensions varies depending on the flow conditions to which it is submitted. Moreover, they are memory fluids. Many examples in the scientific literature show that it is almost impossible to predict the behavior of a concentrated suspension in a metering nip by either a standard viscosity measurement or a viscosity evaluation other than an in-situ measurement.
The present invention is aimed at satisfying the need of being capable to accurately determine the process viscosity of a fluid in a film metering device and while the device is in use. The system and the method of the present invention allows the fluid to be monitored in a continuous manner, giving direct and immediate information about the process viscosity as well as the apparent shear rate of the fluid. More particularly, the present invention provides a method for determining the process viscosity (xcexc) of a fluid supplied to a film metering device. The film metering device includes a transfer roll and a metering rod. A metering nip is formed between the transfer roll and the metering rod. The metering rod has an outer cylindrical surface and is disposed parallel to the transfer roll. The method comprises the steps of obtaining a pressure profile of the fluid, representing the pressure of the fluid in the metering nip on the surface of the metering rod; measuring a value of a torque (T) applied on the metering rod when the pressure profile is taken; determining a maximum pressure value (Pmax) and a minimum pressure value (Pmin) from the pressure profile and respective positions (Xmax, Xmin) thereof; determining a value of a circumferential spacing (L) between the position (Xmax) of the maximum pressure value (Pmax) and the position (Xmin) of the minimum pressure value (Pmin); and calculating the process viscosity (xcexc) of the fluid using the values of the torque (T), the maximum pressure (Pmax), the minimum pressure (Pmin) and the circumferential spacing (L).
The present invention also provides a system for determining the process viscosity (xcexc) of a fluid supplied to a film metering. The film metering device includes a transfer roll and a metering rod, both forming between them a metering nip. The metering rod has an outer cylindrical surface and is disposed parallel to the transfer roll. The system comprises first means for obtaining a pressure profile of the fluid representing the pressure of the fluid in the metering nip on the surface of the metering rod; second means for measuring a value of a torque (T) applied on the metering rod when the pressure profile is taken; third means for determining a maximum value (Pmax) and a minimum value (Pmin) from the pressure profile and relative positions (Xmax, Xmin) thereof; fourth means for determining the value of a circumferential spacing (L) between the position (Xmax) of the maximum pressure (Pmax) and the position (Xmin) of the minimum pressure (Pmin); and fifth means for calculating the process viscosity (xcexc) of the fluid using the values of the torque (T), the maximum pressure value (Pmax), the minimum pressure value (Pmin) and the circumferential spacing (L).
The present invention also provides a system for determining the process viscosity (xcexc) of a fluid supplied to a film metering device. The film metering device includes a transfer roll and an adjacently-disposed metering rod, both forming between them a metering nip. The metering rod has a cylindrical outer surface and a rotation speed (xcfx89). The system comprises a pressure sensor flush-mounted on the outer surface of the metering rod. The pressure sensor generates a first signal (P) indicative of the pressure thereon. A torque sensor is connected to the metering rod. The torque sensor generates a second signal (T) indicative of the torque applied on the metering rod. A computer is provided to record and process data. The computer calculates the process viscosity (xcexc) based on the first signal (P), the second signal (T) and the rotation speed (xcfx89).